Samuel J. Dunham
Astronomy PhD Candidate

About Me

Head shot

Hi! My name is Sam, and welcome to my website! I am a third-year graduate student at Vanderbilt University, studying astronomy. This is the first webpage I've made since high-school, so if you have any comments/criticims or whatever, please let me know!

My research-related interests span a wide range of topics, but include computer simulations, hydrodynamics, and general relativity. Fortuately for me I found the perfect project to work on!

My non-research-related interests span an even wider range of topics, but include cooking, being a dog dad (to Daisy and Lily), enjoying the outdoors, and traveling. I am also a big advocate for animal welfare, especially stopping animal cruelty.

If you're looking for my CV, click here.

Core-Collapse Supernova Research

Image of supernova

The stability of stars is due to the force of gravity being balanced by the pressure from the gas and radiation produced by nuclear fusion in the stellar core. When a star runs out of material to fuse this balance is lost, gravity takes over, and the star begins to collapse. Stars more massive than about eight suns end their lives with a tremendous explosion known as a supernova. The energy output of this explosion is approximately 100 times the energy that our sun will produce in its entire ten-million year lifetime! What's more, it is all released in a matter of seconds! These explosions distribute many of the heavy elements in the solar system, including carbon, the element on which life as we know it is based. So it behooves us to understand this process well, because it is to that which we literally owe our entire existence! Although there are many promising leads, it is not currently known exactly how the explosion proceeds.

What is known is that the core collapses until the pressure is so high that the repulsive electrical forces between the constituent atoms are overcome and the core transforms from a ball of iron into effectively one giant atomic nucleus--this is what will eventually become a neutron star (assuming it doesn't collapse to a black hole). This transition produces a shock wave that propagates outward. It is intuitive that this shock wave simply propagates through the entire star, blowing the material away with it. However, as often happens, nature is not so simple. It was discovered via computer simulations that the shock wave stalls about 200 km from the center of the star. The shock is somehow re-energized, and continues on its explosive path. The goal of the research group I'm in is to determine this re-energizing mechanism.

Currently I collaborate with Profs. Eirik Endeve and Anthony Mezzacappa at Oak Ridge National Laboratory on the toolkit for high-order neutrino-radiation hydrodynamics, thornado, a code that aims to numerically simulate core-collapse supernovae in three-dimensions. My work focuses on developing a module that solves the hydrodynamics equations to make them consistent with a 3+1 decomposition of general relativity under the conformally-flat approximation. To accomplish this we are using a discontinuous Galerkin method for spatial discretization and strong-stability-preserving Runge-Kutta methods for temporal discretization.

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Strong Lensing Research

Image of smiley face lens

One consequence of Einstein's theory of general relativity is that as light travels through a gravitational field, it is deflected. This phenomenon is known as gravitational lensing, because the effects are similar to those of light traveling through a lens. For these effects to be measurable, enormous masses are required, e.g. a star, a galaxy, or something even bigger! One source of measurable gravitational lensing is the light from entire galaxies being deflected by the mass of a galaxy cluster, the largest gravitationally-bound objects in the Universe. In analogy with a physical lens, gravitational lenses can also have magnifiaction effects, increasing the apparent size of a galaxy, allowing astronomers to see features that would otherwise be too small to see.

A special case of gravitational lensing is so-called strong lensing, where the light is deflected to such a degree that multiple light rays leaving the same point, but traveling in different directions, can all be bent directly toward us, causing us to see multiple images of the source (like the mouth and sides of the smiley face in the accompanying picture).

As an undergraduate I did research with Professor Keren Sharon in strong gravitational lensing, as part of the Sloan Giant Arcs Survey (SGAS). My work involved modeling these strong lenses (i.e. clusters of galaxies, two of which make up the eyes of the smiley face in the picture) to determine their masses based on the color, shape, and other properties of the multiple images.

See my undergraduate honor's thesis on one particular lens, SDSS J1438+1454, here, or the paper that came out of that research here.

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